This invention relates to hydrostatic power transmissions, and more particularly it is concerned with a control system for a hydrostatic power transmission comprising a variable-displacement hydraulic pump driven by a prime mover and a hydraulic actuator for driving a load which are connected to each other in a closed or semi-closed circuit.
Heretofore a hydrostatic power transmission, comprising a variable-displacement hydraulic pump driven by a prime mover and a hydraulic actuator for driving a load, the pump and actuator being connected to each other in a closed circuit or a semi-closed circuit, has been known in the art of hydraulically operated machines such as bulldozers, hydraulic shovels, hydraulic cranes, etc. The hydraulic pump is provided with means for adjusting the hydraulic pump displacement in accordance with a manipulated variable of manual operating means or an operating lever. A swash-plate pump of the reversible tilt type is used, for example, as a variable-displacement type hydraulic pump.
The means for adjusting the hydraulic pump displacement usually includes a hydraulic pump displacement adjusting mechanism having a hydraulic piston-cylinder unit with a piston rod operatively connected to a swash-plate of the hydraulic pump, and a servo valve connected between the hydraulic pump displacement adjusting mechanism and a hydraulic fluid source, the servo valve being mechanically linked to the operating lever and associated with the hydraulic pump displacement adjusting mechanism to enable the movement of the mechanism to be mechanically fed back to the servo valve.
In a hydrostatic power transmission of the closed circuit type, a hydraulic motor is usually used as a hydraulic actuator, and an auxiliary pump is mounted therein for merely supplying the hydraulic fluid to compensate for leaks of the hydraulic fluid from a main circuit.
In a hydrostatic power transmission of the semi-closed circuit, a hydraulic cylinder is usually used as a hydraulic actuator, wherein, when the hydraulic cylinder is actuated, the difference between the amount of supply of the working fluid and the amount of discharge thereof, which is produced by the difference in volume between the supply side and the discharge side of the cylinder, is discharged through a flushing valve from the main circuit.
In this type of hydrostatic drive system, however, a sudden actuation of the operating lever would cause a sudden increase in the amount of fluid delivered by the hydraulic pump, and a circuit pressure would become inordinately high due to the inertia of the load driven by the hydraulic actuator. This tendency would be marked when the inertia of the load is high. To avoid this phenomenon, crossover relief valves are connected between conduits of the main circuit for releasing the difference between the delivery by the hydraulic pump and the suction by the hydraulic actuator. This relief of the working fluid entails a loss of energy.
To avoid this loss of energy, a proposal has been made to use a circuit pressure control system in "MACHINE DESIGN", pages 114-116, published on Oct. 7, 1976. This system comprises a three-way change-over valve mounted between the pressure fluid inlet of the servo valve connected to the hydraulic pump displacement adjusting mechanism and the pressure fluid source. The servo valve has a spring mounted in one pilot section thereof while allowing the circuit pressure of the electrostatic power transmission to act on the other pilot section, so that when the circuit pressure rises above a value set by the spring at the time of acceleration of the hydraulic actuator, the three-way change-over valve is actuated by the rise of the circuit pressure to reduce the volume of the pressure fluid supplied to the hydraulic pump displacement adjusting mechanism and reduce the rate of increase of the delivery by the hydraulic pump, thereby to avoid an excessively high rise of the circuit pressure above the value set by the spring. Thus the phenomenon of the excess pressure fluid being released from the main circuit through the crossover relief valve to cause a loss of energy can be avoided.
The aforesaid circuit pressure control system of the prior art is able to effect control in such a manner that the delivery by the hydraulic pump is proportional to the manipulated variable of the operation lever but is unable to so control the delivery by the hydraulic pump wile controlling the maximum value of the circuit pressure in accordance with the manipulated variable of the operation lever.
When a machine is manually operated, the need arises not only to control the speed of the machine, but also to control acceleration or force, depending on the circumstances. For example, when a swiveling member of a hydraulic shovel is driven, actuation of the operating lever over a wide range is motivated by two wishes of the operator, one wish representing a desire to abruptly accelerate the swiveling member and the other wish representing a hope that the swiveling member would move at high speed. When a bucket of the hydraulic shovel carries out excavation while being pressed laterally against earth, the operation can be performed more readily if the pressing force is reduced when the operating lever is pulled a short distance and the pressing force is increased when the operating lever is pulled a long distance. Meanwhile, when a negative load (active load) is applied to a hydraulic actuator and the hydraulic actuator performs a pumping function, it is desirable that the circuit pressure be controlled to be kept at a suitably high pressure level at all times irrespective of the manipulated variable of the operation lever, because power recovery efficiency can be improved and the hydraulic rigidity of the hydraulic power transmission is increased to thereby provide improvements in operability of the hydraulic actuator.
Such control has, however, been difficult to effect with the aforesaid circuit pressure control system of the prior art.